In recent years audible fire alarm signals have standardized patterns, set by the American National Standards Institute (ANSI). For example, the pattern used for smoke alarms, in accordance with ANSI S3.41, is a three-pulse pattern, known as T3, which comprises three half second on pulses, each followed by a half second off period, the set followed by a one and a half second pause, with the cycle repeated for a minimum of 180 seconds. Carbon monoxide detectors use a similar pattern using four pulses, as defined by the National Fire Protection Association (NFPA) referred to as T4, where the signals consist of four 100 milliseconds on pulses, each followed by a 100 millisecond off period, the set followed by a 5 second pause. The alarms may use the older 3100 Hz sine wave or the newer 520 Hz square wave.
The purpose of the acoustic alarm is to alert personnel on site to evacuate, but it is desirable to automatically detect the existence of the acoustic alarm signal so that appropriate action can be taken, such as alerting off site personnel, without requiring integration with the smoke of carbon monoxide detector. Such acoustic detectors exist, but are limited in detection distance and noise suppression, and are prone to false alarms. Examples of prior art detection systems include U.S. Pat. Nos. 7,015,807 and 8,269,625, the entire contents of each of which are incorporated herein by reference.